A heart block is a situation when the electrical signal from top heart chambers that makes the heart contract and beat reaches the bottom heart chambers with delay or not at all. There are three degrees of heart block and symptoms usually appear only in the third or complete heart block. Patients with complete heart block may present with:
- Dizziness and blackouts
- Tiredness and breathlessness
- Palpitations (irregular heartbeat)
Electrical system of the heart
Sinoatrial node (SA node, specialized bundle of heart muscle cells) is the heart’s natural pacemaker. SA fires in regular intervals electrical impulses that pass through atria (top heart chambers) to the atrioventricular node (AV node). AV node is the only electrical connection between atria and ventricles (bottom heart chambers). Electrical impulses then pass through the AV node and via specialized fibers (His bundle and Purkinje fibers) to the ventricles.
Heart block (atrioventricular block) occurs when there is an delay or interruption in the transmission of electrical impulses from AV node to the ventricles. Heart block normally does not lead to cardiac arrest but the back-up (escape) rhythm is usually slow and leads to slow heart beat (bradycardia) and symptoms as described above. Heart block is diagnosed with ECG, in case of intermittent episodes 24-hour ECG monitor may be needed.
Types of heart block
- First degree heart block (atrioventricular, AV block)
- Second degree heart block
- Third degree heart block
These are described in more detail below.
First degree heart block
A first degree AV block means only a minor delay in conduction between atria and ventricles. It does not necessarily mean any cardiac problem, first degree AV block normally does not cause any problems and does not require any treatment.
Second degree heart block
In this case some but not all impulses from atria do not reach ventricles which leads to skipped beats. There are further two subtypes of the second degree AV block:
- Wenckebach or Mobitz type 1 – Increasing delay in AV conduction which eventually leads to a skipped beat when an impulse does not reach ventricles. It is a less serious condition which usually does not require any treatment and in some cases can be entirely physiological.this is the least serious type of second degree heart block; it may occasionally cause mild dizziness and does not usually require treatment.
- Mobitz or Mobitz type 2 – Suddenly skipped beat without previous prolongation in AV conduction. It is the more serious type of second degree AV block which can cause symptoms and progress to the third degree AV block and therefore needs treatment.
Third degree heart block
Third degree AV block is also called complete heart block because the electrical impulses from atria do not reach ventricles. Third degree AV block can be congenital (present from birth, very uncommon) or acquired (develops later in life, relatively common in elderly people). Third degree AV block usually generates symptoms of tiredness, breathlessness, dizziness and blackouts and in some cases it can be a life-threatening emergency requiring prompt treatment with pacemaker.
Treatment and prognosis
First degree AV block and second degree Mobitz I AV block normally don’t require treatment and can be entirely normal in athletic persons. Second degree Mobitz II AV block may not generate much symptoms but can progress unpredictably to the third degree AV block which can present with quite dramatic symptoms and lead to sudden death.
Treatment of Mobitz II AV block and complete heart block is permanent pacemaker implantation. Pacemaker reconnects electrically atria and ventricles and restores normal coordination between heart chambers. Pacemaker insertion in these indications leads to normal quality of life and excellent prognosis.